Linear Interconnect Networking (LIN) is an industry standard for a single-wire serial communication protocol, based on the common serial communication interface (SCI) (UART) byte-word interface. UART interfaces are now available as a low cost silicon module and are provided as a feature on the majority of micro-controllers. UART interfaces can take many forms, for example they can be implemented in software or as a state machine interface for application specific integrated circuits (ASICs).
LIN is targeted as an easy to use, open, communication standard, designed to provide more reliable vehicle diagnostics. Access to the communication medium in a LIN network is controlled by a master node, so that no arbitration or collision management software or control is required in the slave nodes, thus providing a guarantee of worst-case latency times for signal transmission.
A node in a LIN network does not make use of any information about the system configuration, except for the denomination of the master node. Nodes can be added to the LIN network without requiring hardware or software changes in other slave nodes. The size of a LIN network is typically under twelve nodes, although the LIN network is not generally restricted to twelve nodes. This results from a use of only ‘64’ identifiers together with a relatively low transmission speed of 20 Kbits/sec. The clock synchronization, the simplicity of UART communication, and the single-wire medium are often cited as major factors for the cost efficiency of LIN.
Referring now to FIG. 1, a simplified LIN node 100 is illustrated. FIG. 1 shows the basic block diagram of the LIN physical layer. A digital input, referred to as txd 105, drives the transmit (Tx) LIN bus driver 110. When the digital input txd 105 is at high logic level, the LIN output, on the single communication line LIN communication bus 115, is at a high level, i.e. the supply voltage of the vehicle battery referred to as Vbat.
The signal voltage swing on the single communication LIN bus swings from Vbat to a low level of approximately 1V. The Tx LIN bus driver 110 is supplied by Vbat. Each receiver element in a LIN network comprises a comparator 120, which detects when the voltage signal on the single communication LIN bus crosses a value of 50% of Vbat. The voltage level of the comparator output is therefore controlled by the reference signal 125 input to the comparator 120. When the voltage on the single communication LIN bus is high, i.e. over a level of 50% of Vbat, the receiver logic (rxd) output 130 is at a high (Vbat) logic level.
Referring now to FIG. 2, A LIN network 200 is illustrated. The LIN network 200 comprises one master node (control unit) 205 and one or more slave nodes 220, 230. All nodes include a slave communication task 215, 225, 235 that is divided between a transmit task and a receive task. The master node 205 also includes a transmit task 210 and a receive slave task 215. Communication in an active LIN network is performed on the LIN bus 240 and is always initiated by a master task 210.
Referring now to FIG. 3, the simplified circuit of a node is illustrated. FIG. 3 illustrates the output stage of the Tx bus driver 110. The output stage is connected to Vbat 305 through a diode 310. A resistive load 315 is used as a pull-up function for the output stage, i.e. the single LIN communication bus 115. A typical value for a resistive load 315 of a slave device is 30 Kohm. Thus, the 30 Kohms pull-up resistor (in series with diode 310 and located inside the IC and identified as mandatory in the LIN specification) is present in each internal LIN node. However, to distinguish the Master node from a slave node a 1 Kohm resistor is placed in series with another diode (not shown), where both the 1 Kohm resistor and diode are located outside of the integrated circuit.
The transistor 320 functions as a switch, through control of the serial communication interface (SCI) 330, and is therefore able to pull-down the single communication LIN bus 115 to a low level.
The LIN specification demands a very low signal perturbation during a communication. For this reason, it is important to optimize transition between high and low voltage levels on the LIN communication bus. In particular, to avoid creating interference on the LIN signal line, the transition between high and low voltage levels must be smooth.
The LIN standard specifies a maximum transition time for the voltage level to travel between high and low voltage levels. As the LIN bus is a single wire communication bus, the single wire acts as an antenna, which generates radio frequency (RF) interference signals. In this regard, in order to limit the RF interference, the known prior art has focused on employing a voltage transition that is a constant Vbat versus time (ΔV/ΔT) relationship 400, as illustrated in FIG. 4.
Referring now to FIG. 4, the constant Vbat versus time relationship 400 illustrates two typical high voltage level starting positions, e.g. 18V 410 and 12V 455. Of particular note is that the LIN signal may start from a battery voltage of 18V, through either a voltage surge upon switching on the vehicle engine (and a consequent effect on the 12V battery) or using the LIN system in a heavy goods vehicle that uses an 18V battery.
As illustrated, the corresponding slopes 405, 450 transition the voltage to a low voltage level 415. A maximum transition time Tmax 470. is specified from an initial voltage drop from a first threshold voltage (thereby providing a sufficient indication that a voltage transition is occurring) to a second threshold voltage 465, which indicates a low voltage level of operation. Notably, as illustrated, when the voltage transition starts from a high voltage level of 18V, there is insufficient time for the voltage to transition below the low threshold voltage 465. As shown, the 18V starting voltage is only unable to drop 420 within the maximum specified time 470, which notably fails to meet the specifications of the LIN standard.
An alternative approach to transitioning between high and low voltage levels has focused on employing a voltage transition that is a fixed time relationship 500, as illustrated in FIG. 5.
Referring now to FIG. 5, the fixed (constant) time relationship 500 illustrates three typical high voltage level starting positions, e.g. 18V 550, 12V 560 and 7V 540. Of particular note is that the fixed time approach employs a transition mechanism that is not reliant upon the starting high voltage level. Again, the LIN signal may start from a battery voltage of 18V, through either a voltage surge upon switching on the engine (and a consequent effect on the battery) or using the LIN system in a heavy goods vehicle that uses an 18V battery. Alternatively, for poor battery conditions, for example due to cold weather, the high voltage level may only be of the order of, say, 7V.
As illustrated, the corresponding slopes 505, 535 and 545 transition the voltage to a low voltage level 515. Again maximum transition time Tmax570 is specified from an initial voltage drop of one-third (thereby providing a sufficient indication that a voltage transition is occurring) to a level below a threshold, which indicates a low voltage level of operation. Notably, with the lower of the high voltage levels as illustrated, a problem occurs due to an effect of a 0.7V drop across a diode (310 in FIG. 3) in the LIN circuitry and any saturation effect in the associated transistors (320).
To explain this effect in greater detail, a LIN driver node has at least one diode (such as diode 315) between the supply voltage (305 in FIG. 3) and the transistor driver (320 of FIG. 3) and often a second diode (not shown) between the transistor driver and ground. In effect, these diodes plus the drop voltage of the switch, forces the LIN amplitude to be less than Vbat. In other terms, due to the voltage drop across the diode(s), the LIN amplitude is not proportional to Vbat (as VLINpp=Vbat−Vdrop where Vdrop is almost a constant voltage due to the diodes and saturation voltage).
Nevertheless, Vdrop has more influence on the non-proportionality when Vbat is low. However, the Tx-Rx threshold is proportional to Vbat. Hence, due to this non-proportionality, the threshold reported to the LIN signal is changing when Vbat decreases. For example, a 40% to 60% ratio of Vbat, when Vbat=6V, will became a 20% to 80% ratio of the LIN signal. This effect increases the Ttran when Vbat is low. Thus, the fixed (constant) time approach is only truly representative of a constant time, and therefore optimum, when variation in Vbat is negligible compared to the diode voltage drop and Vdrop across the switch.
Thus, the known techniques of employing either the fixed time approach to transitioning the voltage or the constant slope (ΔV/ΔT) approach are both problematic for different reasons. Hence, a need exists for an improved LIN network, integrated circuit and method of operation therefor.